1. Technical Field
The invention relates to plasma reactors having electrostatic chucks for holding a semiconductor wafer on a pedestal inside the vacuum chamber of the reactor, and in particular to a way of protecting the electrostatic chuck from corrosion and erosion by the plasma.
2. Background Art
An electrostatic chuck holds a wafer onto a pedestal inside a plasma reactor chamber using electrostatic attraction. This overcomes the problems of conventional metal clamp rings used for the same purpose, such problems including the masking of the wafer periphery by the metal ring, metal-to-semiconductor contact problems with particulate contamination and the susceptibility of the metal ring to etch damage from the plasma.
However, the electrostatic chuck itself is susceptible to etch damage from the plasma, particularly when employed in a plasma etch process. This is because the electrostatic chuck typically consists of a conductor such as a copper film covered top and bottom by an insulator film such as polyimide, which is susceptible to being removed by the plasma. In order to hold the wafer to the chuck, the copper layer is held at a high voltage (e.g., 600 volts D.C.). If the plasma removes enough polyimide insulation from the copper conductor to short the conductor to the plasma, there is insufficient electrostatic force to hold the wafer to the chuck.
Another problem is that in a typical electrostatic chuck, the wafer periphery overhangs the edge of the chuck, so that any plasma escaping beyond the edge of the wafer not only etches the electrostatic chuck, as discussed above, but also etches the backside of the wafer periphery, an undesirable effect.
The foregoing problems with the electrostatic chuck arise primarily because there must be at least a small (1-3 mm) circumferential gap between the wafer edge and the side wall of the chamber. Typically, the chamber side wall, in certain reactor types, is quartz and is consumed slowly over the course of processing many wafers to provide a material for scavenging fluorine in a plasma etch process. It is through the circumferential gap between the quartz side wall and the wafer periphery that the plasma can extend to the region behind the wafer, so as to corrosively attack the electrostatic chuck and the wafer periphery backside.